In an optical transmission system, which carries out Raman amplification, the loss caused in an optical transmission channel or in the optical parts on the optical transmission channel can be corrected by adjusting the Raman gain. The correction of the loss is advantageous, because, as compared to an optical amplification transmission system that employs only a lumped parameter style optical amplifier, it is possible to maintain the required signal-to-noise ratio during the transmission and thereby transmit the same signal to a longer distance.
However, the Raman gain is polarization-dependent. One approach, to solve this problem, is to use a non-polarized pump light. A linearly-polarized pump light may be non-polarized by polarization combining (see, for instance, “optical transmission system” disclosed in the Japanese Patent Laid-Open Publication No. 2000-151507) or by using a polarization dispersion device (see, for instance, “laser diode module and depolarizer” disclosed in the Japanese Patent Laid-Open Publication No. H8-254668).
In the polarization combining, it is necessary to provide two pump light sources that transmit light beams of an almost equal intensity. Moreover, to improve the wavelength characteristics of the Raman gain, it is desirable to use two or more pump lights that have different central wavelengths. In other words, in the polarization combining, two light sources are necessary for each pump wavelength. Thus, it is necessary to provide the pump light sources two times of the number of pump wavelength.
It is common in the polarization dispersion device to use a semiconductor laser module (LD) as an optical source. Such a semiconductor laser module generally includes a wavelength stabilizing fiber grating as a typical pump light source used in the Raman amplification of a light signal that has a wavelength in the range of 1.55 micrometers (μm) and forms an external resonator. Moreover, such a semiconductor laser module has a central wavelength of 1430 nanometers (nm), a full width at half maximum of an optical spectral envelope of 145 gigahertz (GHz), a vertical mode interval of the semiconductor laser module device of 33 GHz, and a full width at half maximum of the vertical mode of the semiconductor laser module device of 10 GHz.
When the polarization dispersion device is used, lesser pump light sources are required as compared to those required in the polarization combining. However, in case of the polarization dispersion, even if an optical path difference, which is longer than a coherent length (about 1 millimeter (mm)) corresponding to the full width at half maximum of the optical spectral envelope, is assigned between two polarization modes, the possibility of coherency still remains. As a result, sometimes polarization of the pump light cannot be eliminated so that it becomes necessary to assign an optical path difference longer than the coherent length (about 2 centimeters (cm)) corresponding to the vertical mode of the LD device.
Assuming that a polarization maintaining fiber having polarization dispersion of about 1.4 picoseconds (ps) in 1 meter (m) is used, the optical path difference between the two polarization modes is about 0.3 mm. Therefore, the polarization maintaining fiber has to be 70 m or longer. This poses a problem in terms of cost and the implementation capacity.
It is an object of the present invention to solve at least the problems in the conventional technology.